The present invention relates to a method for separating isomers and different conformations of ions in gaseous phase, based on the principle of high field asymmetric waveform ion mobility spectrometry.
High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents (see, for example, G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and Plasma Chromatography, edited by T. W. Carr (Plenum, New York, 1984)). In ion mobility spectrometry, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are gated into the drift tube and are subsequently separated based upon differences in their drift velocity. The ion drift velocity is proportional to the electric field strength at low electric fields (e.g., 200 V/cm) and the mobility, K, which is determined from experimentation, is independent of the applied field. At high electric fields (e.g. 5000 or 10000 V/cm), the ion drift velocity may no longer be directly proportional to the applied field, and K becomes dependent upon the applied electric field (see G. Eiceman and Z. Karpas, Ion Mobility Spectrometry (CRC. Boca Raton, Fla. 1994); and E. A. Mason and E. W. McDaniel, Transport Properties of Ions in Gases (Wiley, New York, 1988)). At high electric fields, K is better represented by Kh, a non-constant high field mobility term. The dependence of Kh on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS), a term used by the inventors throughout this disclosure, and also referred to as transverse field compensation ion mobility spectrometry, or field ion spectrometry (see I. Buryakov, E. Krylov, E. Nazarov, and U. Rasulev, Int. J. Mass Spectrom. Ion Proc. 128. 143 (1993); D. Riegner, C. Harden, B. Carnahan, and S. Day, Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, Calif., Jun. 1-5, 1997, p. 473; B. Carnahan, S. Day, V. Kouznetsov, M. Matyjaszczyk, and A. Tarassov, Proceedings of the 41st ISA Analysis Division Symposium, Framingham, Mass., Apr. 21-24, 1996, p. 85; and B. Carnahan and A. Tarassov, U.S. Pat. No. 5,420,424). Ions are separated in FAIMS on the basis of the difference in the mobility of an ion at high field Kh relative to its mobility at low field K. That is, the ions are separated because of the compound dependent behaviour of Kh as a function of the electric field. This offers a new tool for atmospheric pressure gas-phase ion studies since it is the change in ion mobility and not the absolute ion mobility that is being monitored.
An instrument based on the FAIMS concept has been designed and built by Mine Safety Appliances Company of Pittsburgh, Pa. (xe2x80x9cMSAxe2x80x9d) for use in trace gas analysis. The MSA instrument is described in U.S. Pat. No. 5,420,424 and is available under the trade mark FIS (for Field Ion Spectrometer). While the use of the MSA instrument (and similar instruments based on the FAIMS concept) for trace gas analysis is known, the inventors believe that they have identified certain heretofore unrealized properties of these instruments which make them more versatile. Based on this realization, the inventors have developed what is believed to be a previously unknown method for separation of isomers and different conformations of ions. A summary and detailed description of the present invention is provided below.
The present invention provides a method for identifying ions having substantially the same mass to charge ratio but having different ion mobility characteristics, comprising the steps of:
a) providing at least one ionization source of ions;
b) providing an analyzer region defined by a space between at least first and second spaced apart electrodes, said analyzer region being in communication with a gas inlet, a gas outlet, an ion inlet and an ion outlet, and introducing said ions into said analyzer region through said ion inlet;
c) applying an asymmetric waveform voltage and a direct current compensation voltage to at least one of said electrodes;
d) setting said asymmetric waveform voltage;
e) varying said direct current compensation voltage and measuring resulting transmitted ions at said ion outlet, so as to produce a compensation voltage scan for said transmitted ions; and
f) identifying peaks in said compensation voltage scan.
The method may further comprise the step of setting said direct current compensation voltage to correspond to one of said peaks to separate a desired ion from other ions with substantially the same mass to charge ratio.
Advantageously, the above method is operable at substantially at atmospheric pressure and substantially at room temperature.
The method may further include detecting said transmitted ions by mass spectrometry.
Typically, the method includes providing a gas flow through said analyzer region, so as to transport said ions along said analyzer region, although it will be understood that other ion transport means are possible.
Furthermore, in identifying a peak, it will be understood that the term peak is not limited to the apex of the peak, and that a peak will typically have a noticeable width, or a compensation voltage range in which the peak appears.